1. Field of the Invention
This invention relates to an adsorptive hydrocarbon separation process using crystalline zeolites. More specifically, the invention pertains to the selective separation of non-aromatic compounds in admixture with aromatic compounds over a particular crystalline zeolite which selectively sorbs the non-aromatic compounds from the mixture. The zeolites found to be useful in the present process are those having a silica/alumina mole ratio of greater than about 12, a Constraint Index of between about greater than 2 and about 12, and in particular zeolites ZSM-23 and ZSM-35.
2. Description of the Prior Art
It has long been known that certain porous substances such as silica gel, activated char, and zeolites, have certain selective adsorption characteristics useful in resolving a hydrocarbon mixture into its component parts. Thus, silica gel is selective in removing aromatic hydrocarbons from non-aromatic hydrocarbons and activated chars are useful in separating olefins from mixtures with paraffins. Similarly, it is well known in the art that certain crystalline zeolites can be used to separate certain hydrocarbons from feed mixtures.
The selective sorption properties of zeolites are generally known and have been described, for instance, in U.S. Pat. Nos. 2,850,549; 2,866,835; 3,037,338 and 3,218,367. The general sorption properties of zeolites have been disclosed in some of the earlier patents on the zeolites per se, namely U.S. Pat. Nos. 2,882,243 and 2,882,244. Additionally, there are numerous literature references, especially those of Barrer, which deal extensively with the sorption properties of crystalline zeolites. Generally speaking, crystalline zeolites are shape-selective in that they will admit molecules of specific geometry while excluding other molecules.
The separation of normal paraffins from branched chained paraffins for example can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules. U.S. Pat. Nos. 3,265,750 and 3,510,423 for example, disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons from non-olefinic hydrocarbons. Processes to separate straight chain hydrocarbons from a mixture of straight chain and non-straight chain hydrocarbons using a molecular sieve selective adsorbent are described in U.S. Pat. Nos. 3,619,409 and 3,619,416.
Additionally, such crystalline zeolites will exclude aromatics such as benzene while admitting normal hexane. It has been disclosed in British Pat. No. 600,453 that zeolites can be employed as selective sorption agents and that such zeolites will sorb polar molecules in preference to less polar molecules. A method for selectively sorbing a compound of low polarity in admixture with a compound of greater polarity using a zeolite is disclosed in U.S. Pat. No. 3,732,326. The latter patent discloses use of zeolite ZSM-5 for selectively sorbing hydrocarbons of low polarity in admixture with compounds of greater polarity such as water, alcohols, acids, aldehydes and halogen-substituted compounds.
U.S. Pat. No. 3,723,302 discloses a process for separating olefins from a feed stream containing olefins and paraffins using type X or type Y zeolites. A process for the separation of olefins from a hydrocarbon feed mixture using a zeolite absorbent is disclosed in U.S. Pat. No. 3,969,223. A process for the separation and recovery of hydrocarbons selected from paraffins or olefins or both from admixture with aromatic hydrocarbons using aluminum-deficient mordenite is disclosed in U.S. Pat. No. 3,485,748.
The separation of xylene isomers has received a great deal of attention. This interest is generally attributed to the usefulness of para-xylene in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron," "Mylar" and "Terylene." Mixtures of xylene isomers generally contain a concentration of about 24 weight percent para-xylene in the equilibrium mixture. Processes to separate xylene isomers include low temperature crystallization, fractional distillation, selective sulfonation with subsequent hydrolysis and selective solvent separation. Such processes, however, have involved high operation costs and usually result in a limited yield.
U.S. Pat. No. 3,868,429 discloses a method to separate xylene isomers by using activated carbon.
The separation of xylene isomers by the use of faujasite zeolites (type X and type Y zeolites) has been extensively studied. The use of type X and type Y zeolites in xylene isomer separation and similar separations is disclosed in U.S. Pat. Nos. 3,114,782; 3,126,425; 3,133,126; 3,558,730; 3,558,732; 3,626,020; 3,663,638; 3,665,046; 3,686,342; 3,943,183 and 4,051,192.
U.S. Pat. No. 3,793,385 discloses a process for the separation of aromatic isomers, more particularly xylene isomers, by using zeolite beta.
U.S. Pat. No. 3,724,170 discloses chromatographic separation of C.sub.8 aromatic mixtures over zeolite ZSM-5. U.S. Pat. No. 3,699,182 discloses use of zeolite ZSM-5 in a process for selective separation of biphenyls from mixtures containing same and para-disubstituted aromatic isomers from mixtures containing same. British Pat. No. 1,420,796 shows use of zeolite ZSM-5 for adsorptive separation of p-xylene and ethylbenzene from a mixture comprised of the xylene isomers and ethylbenzene.
Catalytic dewaxing of gas oil fractions over the shape selective zeolites ZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38 is taught in U.S. Pat. Nos. 3,980,550 and 4,149,960.
The ZSM-5 class of crystalline zeolites has been shown to be catalytically selective. This shape selectivity can be further enhanced by the use of very large crystals, impregnation with Mg and P to reduce zeolite pore openings and coke selectivation. These modified zeolite catalysts have been very effective in such reactions as selective toluene disproportionation which yields predominantly paraxylene as the product and toluene-ethylene alkylation yielding primarily para-ethyltoluene.
Zeolite ZSM-5 possesses pore openings intermediate in size between the small pore and the large pore zeolites. It sorbs at room temperature straight chain monomethyl-substituted paraffins and monocyclic hydrocarbons at significantly faster rates than those containing dimethyl-substituted or quaternary carbon atoms, and it excludes molecules with critical dimensions larger than that of 1,3,5-trimethylbenzene. Zeolite ZSM-5 has a pore system which differentiates catalytically molecules having a straight chain, a methyl substitution and a dimethyl substitution. The catalytic properties of ZSM-5 are further elucidated by Chen and Garwood in Some Catalytic Properties of ZSM-5, a New Shape Selective Zeolite, JOURNAL OF CATALYSIS, Vol. 52, No. 3 (May 1978).
Satterfield and Cheng, Liquid Sorption Equilibrium of Selected Binary Hydrocarbon Systems in Type Y Zeolites, AICHE JOURNAL, Vol. 18, No. 4, p. 720, July 1972 and Satterfield and Smeets, Liquid Sorption Equilibria of Selected Binary Paraffin Systems in NaY Zeolite, AICHE JOURNAL, Vol. 20, No. 3, p. 618, May 1974, teach that on zeolite Y aromatic compounds are selectively adsorbed over paraffins and smaller compounds are adsorbed in preference to larger compounds. Contrary to said teaching, the zeolites for use in the instant invention yield the unexpected results of selective adsorption of paraffins over aromatics and selective adsorption of higher molecular weight molecules over smaller members of the same family.